This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-275538, filed September 29, the entire contents of which are incorporated herein by reference.
The present invention relates to an image forming apparatus applied as a color printer apparatus or a color copying apparatus, as well as an image forming method.
An example of the image forming apparatus of the above-described type is a 4-series tandem type apparatus in which a plurality of electronic photographic process units are arranged to oppose to the same transfer belt.
These electronic photographic process units are arranged to have predetermined intervals therebetween along the direction in which sheets are conveyed, and the transfer belt is designed to swing in a swaying direction to be brought into contact with or away from the photosensitive drums of the respective electronic photographic process units in accordance with a printing mode. These plurality of photosensitive drums are used for colors of yellow, magenta, cyan and black.
In an image forming operation, when, for example, a full-color printing mode is designated, the transfer belt is tilted towards the photosensitive drums and is brought into rotation-contact with all of the photosensitive drums.
Sheets are fed from a feeder portion, and then each of them is aligned by a pair of aligning rollers, to be output to the transfer belt. A fed sheet is conveyed to each of the photosensitive drums one after another, and thus toner images of these colors are transferred on the sheet one on another.
When a black and white printing mode is designated, the transfer belt is tilted in an opposite direction to the case of the full-color mode, with respect to the rotation-contact section between the transfer belt and the photosensitive drum for black, situated at the lowermost stream in the sheet conveying direction, so as to separate it from each of the photosensitive drums for yellow, magenta and cyan. Therefore, the sheet conveying power of the transfer belt decreases. Therefore, a charger is provided below a sheet introduction side of the transfer belt so as to charge the transfer belt electrostatically by the charger. By means of the electrostatic charge, each sheet is attracted to the transfer belt so that it can be conveyed surely to the photosensitive drum for black.
Further, a press roller which is also brought into rotation-contact with the transfer belt is provided above the sheet introduction side of the transfer belt. With this structure, in the case of the black and white printing mode, a sheet is pressed against the transfer belt by the press roller, so as to assure the sheet to be appropriately conveyed.
However, conventionally, in the black and white mode, when the both-side printing mode is designated, a sheet printed on one side is inversed and then guided to the aligning roller pair. During this operation, a straight sheet is given a curling shape, and thus it is easily distorted that way at the inversion site, and as a result, the sheet is not completely or entirely attached to the transfer belt, creating a portion partially floating from the belt. If a sheet partially floats from the belt, it is likely to have a problem of the sheet being stuck on the photosensitive drum, which causes the increasing of the occurrence rate of paper jam.
Meanwhile, an upper guide plate and a lower guide plate for guiding sheets are provided between the aligning roller pair and the press roller. Further, there is a difference between the feeding speed for sheets fed from feeder means and the conveying speed for sheets conveyed by the transfer belt, and due to this difference in speed, a sheet is deformed to have a warp. The warp deformation of the sheet is imparted within a space defined by the aligning roller pair, the press roller and the upper and lower guide plates.
However, with regard to a small-sized color printer or color copying machine, there is a limitation to the size of the device, and the distance between the aligning roller pair and the transfer belt cannot be designed wide. As a result, in the conventional cases, the press roller is situated always at the same position and projects in the space, regardless of the type of the printing mode, and the space is reduced in size, which easily causes the problem of paper jam.
The present invention has been achieved in consideration of the above-described circumstances, and the object thereof is to provide an image forming apparatus capable of accurately conveying an image transfer medium material without having jamming of the image transfer medium material by varying the conveying mode of the image transfer section for the object material in accordance with the image forming state of the object material fed to the transfer section, and capable of having a design in which a large space can be prepared for the object material to warp, as well as such an image forming method.
According to the present invention, there is provided an image forming apparatus comprising a plurality of image forming means for forming developer images of different colors, transfer means for conveying an image transfer medium to the image carriers of the plurality of image forming means, and transferring the developer images formed by the plurality of developer image forming means on the image transfer medium, identification means for identifying developer image forming means necessary for forming a predetermined image, switch means for switching, in accordance with an identification result of the identification means, between a first contact state where the transfer means is brought into contact with a part of the plurality of developer image forming means and a second contact state where the transfer means is brought into contact with all of the plurality of developer image forming means, mode selection means for selecting either one of a first transfer mode for transferring an image on only one side of the image transfer medium, and a second transfer mode for transferring an image on both sides of the image transfer medium, and control means for controlling to set the contact state between the transfer means and the plurality of developer image forming means to the second contact state regardless of the identification result of the identification means in the case where the mode designated by the mode selection means is the second transfer mode.
Further, according to the present invention, there is provided an image forming apparatus comprising a plurality of image forming units for forming images on image carriers, a transfer unit for conveying an image transfer medium to the image carriers of the plurality of image forming units, and transferring the images on the image carriers on one surface side of the image transfer medium, a feeder unit for feeding the image transfer medium to the transfer unit, a contact-separation mechanism for bringing the transfer unit in contact with all of the plurality of image forming units in a multi-color image forming mode, and for bringing the transfer unit in contact with image carrier of some of the plurality of image forming units, in a monochrome image forming mode, a judgment unit for judging whether or not an image is formed on an other surface side of the image transfer medium fed to the feeder unit, and a control unit for controlling an operation of the contact-separation mechanism such as to bring the transfer unit into contact with all of the image carriers of the plurality of image forming units when the judgment unit judges that an image is formed on the other surface side of the image transfer medium in a single color image forming mode.
Furthermore, according to the present invention, there is provided an image forming method of an image forming apparatus having a plurality of image forming means for forming developer images of different colors, and transfer means for transferring the developer images formed by the plurality of developer image forming means on the image transfer medium, the method comprising a first identification step for identifying if a mode is for forming a multi-color image or a monochrome image, a second identification means for identifying a first transfer mode where an image is transferred on only one side of the image transfer medium or a second transfer mode where an image is transferred on both sides of the image transfer medium a switching step for switching, in accordance with identification results of the first and second identification means, contact states between the transfer means and the plurality of developer image forming means.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.